A variety of systems are known in the art for providing drivers with in-vehicle electronic routing maps and navigation aids. These systems are commonly coupled to a location-finding device in the vehicle, such as a Global Positioning System (GPS) receiver. The GPS receiver automatically determines the current location of the vehicle, to be displayed on the map and used in determining routing instructions.
In-vehicle navigation systems fall into two general categories: “on-board” systems, in which the map data are stored electronically in the vehicle (typically on optical or magnetic media); and “off-board” systems, in which the map data are furnished by a remote map server. Off-board systems typically use a client program running on a smart cellular telephone or personal digital assistant (PDA) in the vehicle to retrieve information from the server over a wireless link, and to display maps and provide navigation instructions to the driver.
Various off-board navigation systems are described in the patent literature. For example, the above-mentioned Patent Application Publication US 2004/0030493 A1 describes a method for displaying a map on a mobile client device. Map data, including vector information delineating features in the map, are stored on a server. The server determines a route from a starting point to a destination within an area of the map. The route includes a sequence of route segments, each having a respective length and heading angle. The server then defines a corridor map comprising a sequence of map segments, each of which contains a respective route segment and has a respective zoom level and orientation determined by the length and heading angle of the route segment. The server downloads the vector information in the map segments to the client device, which renders a succession of images of the map segments as the user travels along the route. Typically, each map segment includes crossroads that intersect the route. If the user deviates from the route, the client device displays a return path to the route on one of the crossroads.
As another example, U.S. Pat. No. 6,381,535, whose disclosure is incorporated herein by reference, describes improvements required to convert a portable radiotelephone into a mobile terminal capable of functioning as a navigational aid system. Itinerary requests of the mobile terminal are transmitted to a centralized server by a radio relay link. The server calculates the itinerary requested, and transmits the itinerary to the mobile terminal in the form of data concerning straight lines and arc segments constituting the itinerary. The server also evaluates the possibility of the vehicle deviating from its course and transmits data concerning segments of possible deviation itineraries in an area of proximity to the main itinerary.
Other off-board navigation systems are described in PCT Publications WO 01/01370 and WO 01/27812; in U.S. Pat. Nos. 6,038,559, 6,107,944, 6,233,518, 6,282,489, 6,320,518, 6,347,278, 6,381,535, 6,462,676, 6,43,630 and 6,526,284; and in U.S. Patent Application Publication 2001/0045949. The disclosures of all these patents and publications are incorporated herein by reference.